The present invention relates to the general field of gas turbine engines, and more particularly to the compressors of such engines.
A gas turbine engine comprises a combustion part and a turbine part disposed downstream from a compression part. An annular passage for passing a gas flow extends axially through these various parts of the engine. The gas flow is compressed by the compression part prior to being mixed and burnt with fuel in the combustion part. The gases coming from the combustion part then pass through the turbine part so as to provide propulsion thrust and drive the turbines. The elements of the compression part are constrained to rotate with the turbines by a drive shaft.
The compression part of a gas turbine engine may comprise three axial compressors so as to increase compression of the gas flow: a fan; a low pressure compressor; and a high pressure compressor. Each compressor is typically constituted by a rotary portion (a rotor) a stationary portion (a stator), and a casing. A rotor inner shroud and a stator outer shroud define the radial boundaries of the annular section of the flow of gas passing through the compressor. The rotor comprises a plurality of rows of moving blades which extend radially through the flow section from the inner shroud to the vicinity of the outer shroud. The stator comprises a plurality of rows of stationary vanes extending from the outer shroud, likewise through the flow section between the outer shroud to the inner shroud. Each nozzle-forming row of stationary vanes is disposed between two successive rows of moving blades of the rotor. The stationary vanes of the nozzle serve to guide the gas flow coming from the rows of moving blades to take up appropriate speed and direction. Each stationary vane is constituted by a plurality of vane sections in alignment along a stacking axis and forming the vane profile.
In normal operation of the engine, the rotation of the shaft driving the compression part gives rise to an unbalance phenomenon. The unbalance leads to cyclical loading and vibration that the rotor communicates to the stator of the engine with significant risk of the engine being damaged. In the compressors, this unbalance phenomenon leads to orbital movement of the inner shroud due to its rotation. By the inner shroud making contact with the stationary vanes of the nozzle, this orbital movement is transmitted in the form of radial displacement which has the consequence of deforming the outer shroud to which the vanes are fixed. Furthermore, the fixed nozzle vanes subjected to such radial displacement bend and run the risk of breaking (buckling phenomenon).
In order to avoid excessive deformation of the outer shroud and to avoid breaking the vanes of the nozzle, the nozzle vanes generally have a profile with a C-shaped bend (also known as a sail-shape). Such a shape is characterized by the vane sections situated in the middle of the flow section being tangentially offset relative to lower and upper vane sections that are close to the inner and outer shrouds, thus serving to reduce the buckling strength of the nozzle vanes. A vane constituted by a stack of such sections is more flexible and can therefore absorb a fraction of the deformation energy transmitted by the inner shroud.
Nevertheless, sail-shaped slopes penalize the aerodynamic performance of the compressor, particularly in terms of surge margin. The tangential offset of the vane has the effect of reducing the angles between the blade and the outer and inner shrouds, and beyond a certain value this is aerodynamically penalizing for the compressor. The gas flow passing through the nozzle tends to migrate from the lower and upper sections of the vanes towards the centers thereof. This migration of flow is particularly harmful in terms of surge margin at the base of the vane (bottom sections).